1. Technical Field
This application relates generally to formation fluid analysis as may be accomplished downhole or at a surface. More particularly, this application relates to detection and identification of dew drop out in gaseous samples using patterned surfaces arranged to collect dew, the presence of dew determined by optical interrogation of the patterned surfaces.
2. Background Information
One of the fundamental challenges of measuring the dew point of a live gaseous sample is making an accurate measurement in the presence of filtrate contamination. The filtrate can be water or oil-based, adding a further complication that can be a source of contamination.
A measurement similar to a dew point determination is performed by the fluorescence detector in some formation evaluation tools. In such applications, however, pressure is not well-controlled. The fluorescence signal emerging from droplets condensing on a window abutting a flowline is used as an indication that the dew point has been reached. This signal, however, could potentially originate from fluorescence in the aqueous filtrate as well, producing an incorrect estimation of the dew point of the formation gas. An example is shown in FIG. 1, obtained by surface measurements with PVT Express, an onsite well fluid analysis service commercially available from Schlumberger Technology Corporation, Sugar Land, Tex., USA. PVT Express uses optical properties, such as refractive index, to detect two dew points (e.g., thought to be water and then alkane). The bottom trace originates from a sapphire probe that detects two dew points, one at 1300(s) and the other at about 2750 (s). The curve illustrates two dew points, leading to ambiguity that must be interpreted by the operator. The weakness of the signal should also be noted; the difference between single phase and dew is on the order of 0.03, for example, out of a magnitude of about 2.05, representing a change of only about 1% in signal magnitude.